Astrocytes are complex bushy cells that serve important functions through close contacts between their processes and synapses. However, the spatial interactions and dynamics of astrocyte processes ...relative to synapses have proven problematic to study in adult living brain tissue. Here, we report a genetically targeted neuron-astrocyte proximity assay (NAPA) to measure astrocyte-synapse spatial interactions within intact brain preparations and at synaptic distance scales. The method exploits resonance energy transfer between extracellularly displayed fluorescent proteins targeted to synapses and astrocyte processes. We validated the method in the striatal microcircuitry following in vivo expression. We determined the proximity of striatal astrocyte processes to distinct neuronal input pathways, to D1 and D2 medium spiny neuron synapses, and we evaluated how astrocyte-to-excitatory synapse proximity changed following cortical afferent stimulation, during ischemia and in a model of Huntington’s disease. NAPA provides a simple approach to measure astrocyte-synapse spatial interactions in a variety of experimental scenarios.
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•An imaging method was developed to assess astrocyte-synapse proximity•The method was used to determine the wiring diagram of striatal astrocytes•Dynamics of striatal astrocyte interactions with excitatory inputs were evaluated•Resources to track static and dynamic astrocyte-synapse interactions are provided
The Khakh laboratory and collaborators used state-of-the-art optical and genetic strategies to develop an imaging approach to measure static and dynamic interactions of astrocyte processes with synaptic elements within intact adult brain preparations.
The results of the study of Xu and colleagues in this issue constitute a critical new piece of information on the functional specialization of astrocytes for K super(+) homeostasis in the brain. The ...relationship between astrocytes and potassium has been long recognized in half a century of research. Now this relation appears to have found its metabolic correlate in astrocytic glycogen. Xu et al. showed that glycogen is committed to fuel astrocytic K super(+) uptake, as this process is abolished when glycogenolysis is inhibited even in the presence of glucose. They went further by showing that the cellular mechanisms which selectively mobilize glycogen involve the participation of several intracellular signaling cascades. As with all good science, these findings generate a number of fundamental questions that are open for experimental research.
The importance of astrocytic K super(+) uptake for extracellular K super(+) (K super(+) sub(e)) clearance during neuronal stimulation or pathophysiological conditions is increasingly acknowledged. It ...occurs by preferential stimulation of the astrocytic Na super(+),K super(+)-ATPase , which has higher K sub(m) and V sub(max) values than its neuronal counterpart, at more highly increased K super(+) sub(e) with additional support of the cotransporter NKCC1. Triggered by a recent DiNuzzo et al. paper, we used administration of the glycogenolysis inhibitor DAB to primary cultures of mouse astrocytes to determine whether K super(+) uptake required K super(+)-stimulated glycogenolysis. KCl was increased by either 5 mM (stimulating only the Na super(+),K super(+)-ATPase ) or 10 mM (stimulating both transporters) in glucose-containing saline media prepared to become iso-osmotic after the addition. DAB completely inhibited both uptakes, the Na super(+),K super(+)-ATPase -mediated by preventing Na super(+) uptake for stimulation of its intracellular Na super(+)-activated site, and the NKCC1-mediated uptake by inhibition of depolarization- and L-channel-mediated Ca super(2+) uptake. Drugs inhibiting the signaling pathways involved in either of these processes also abolished K super(+) uptake. Assuming similar in vivo characteristics, partly supported by literature data, K super(+)-stimulated astrocytic K super(+) uptake must discontinue after normalization of extracellular K super(+). This will allow Kir1.4-mediated release and reuptake by the less powerful neuronal Na super(+),K super(+)-ATPase .
Highlights • Astrocytes are fundamental for homoeostasis, defence and regeneration of the CNS. • In Alzheimer’s disease (AD) astrocytes undergo degeneration and atrophy. • Atrophy of astrocytes ...alters homeostatic reserve of the brain. • Astrocytes can be regulated through environmental stimulation and/or medication. • Astrocytes can be targeted for prevention and cure of neurodegenerative disorders.
Astrocytes are key cellular partners for neurons in the central nervous system. Astrocytes react to virtually all types of pathological alterations in brain homeostasis by significant morphological ...and molecular changes. This response was classically viewed as stereotypical and is called astrogliosis or astrocyte reactivity. It was long considered as a nonspecific, secondary reaction to pathological conditions, offering no clues on disease‐causing mechanisms and with little therapeutic value. However, many studies over the last 30 years have underlined the crucial and active roles played by astrocytes in physiology, ranging from metabolic support, synapse maturation, and pruning to fine regulation of synaptic transmission. This prompted researchers to explore how these new astrocyte functions were changed in disease, and they reported alterations in many of them (sometimes beneficial, mostly deleterious). More recently, cell‐specific transcriptomics revealed that astrocytes undergo massive changes in gene expression when they become reactive. This observation further stressed that reactive astrocytes may be very different from normal, nonreactive astrocytes and could influence disease outcomes. To make the picture even more complex, both normal and reactive astrocytes were shown to be molecularly and functionally heterogeneous. Very little is known about the specific roles that each subtype of reactive astrocytes may play in different disease contexts. In this review, we have interrogated researchers in the field to identify and discuss points of consensus and controversies about reactive astrocytes, starting with their very name. We then present the emerging knowledge on these cells and future challenges in this field.
Main Points
Astrocytes react to brain homeostasis alteration by morphological and molecular changes.
This response is complex, heterogeneous and subject to controversies.
New tools, models and concepts will help better understand this widespread reaction.
The control and maintenance of the intracellular redox state is an essential task for cells and organisms. NAD super(+) and NADH constitute a redox pair crucially involved in cellular metabolism as a ...cofactor for many dehydrogenases. In addition, NAD super(+) is used as a substrate independent of its redox-carrier function by enzymes like poly(ADP)ribose polymerases, sirtuins and glycohydrolases like CD38. The activity of these enzymes affects the intracellular pool of NAD super(+) and depends in turn on the availability of NAD super(+). In addition, both NAD super(+) and NADH as well as the NAD super(+)/NADH redox ratio can modulate gene expression and Ca super(2+) signals. Therefore, the NAD super(+)/NADH redox state constitutes an important metabolic node involved in the control of many cellular events ranging from the regulation of metabolic fluxes to cell fate decisions and the control of cell death. This review summarizes the different functions of NAD super(+) and NADH with a focus on astrocytes, a pivotal glial cell type contributing to brain metabolism and signaling.
Astrocytes are a large and diverse population of morphologically complex cells that exist throughout nervous systems of multiple species. Progress over the last two decades has shown that astrocytes ...mediate developmental, physiological, and pathological processes. However, a long-standing open question is how astrocytes regulate neural circuits in ways that are behaviorally consequential. In this regard, we summarize recent studies using Caenorhabditis elegans, Drosophila melanogaster, Danio rerio, and Mus musculus. The data reveal diverse astrocyte mechanisms operating in seconds or much longer timescales within neural circuits and shaping multiple behavioral outputs. We also refer to human diseases that have a known primary astrocytic basis. We suggest that including astrocytes in mechanistic, theoretical, and computational studies of neural circuits provides new perspectives to understand behavior, its regulation, and its disease-related manifestations.
In this review, Nagai et al. summarize recent findings and provide an interpretative framework for diverse astrocyte mechanisms regulating neural circuit functions and animal behavior in multiple species, including C. elegans, Drosophila melanogaster, Danio rerio, and Mus musculus.
Expression of the 17β-estradiol (E2) synthesis enzyme aromatase is highly upregulated in astrocytes following brain injury. However, the precise role of astrocyte-derived E2 in the injured brain ...remains unclear. In the current study, we generated a glial fibrillary acidic protein (GFAP) promoter-driven aromatase knock-out (GFAP-ARO-KO) mouse model to deplete astrocyte-derived E2 in the brain and determine its roles after global cerebral ischemia (GCI) in male and female mice. GFAP-ARO-KO mice were viable and fertile, with normal gross brain structure, normal morphology, intensity and distribution of astrocytes, normal aromatase expression in neurons, and normal cognitive function basally. In contrast, after GCI, GFAP-ARO-KO mice: (1) lacked the normal elevation of astrocyte aromatase and hippocampal E2 levels; (2) had significantly attenuated reactive astrogliosis; and (3) displayed enhanced neuronal damage, microglia activation, and cognitive deficits. RNA-sequencing (RNA-seq) analysis revealed that the ischemic GFAP-ARO-KO mouse hippocampus failed to upregulate the "A2" panel of reactive astrocyte genes. In addition, the JAK-STAT3 pathway, which is critical for the induction of reactive astrogliosis, was significantly downregulated in the GFAP-ARO-KO hippocampus following GCI. Finally, exogenous E2 administration fully rescued the compromised JAK-STAT3 pathway and reactive astrogliosis, and reversed the enhanced neuronal damage and microglial activation in the GFAP-ARO-KO mice after GCI, suggesting that the defects in the KO mice are because of a loss of E2 rather than an increase in precursor androgens. In conclusion, the current study provides novel genetic evidence for a beneficial role of astrocyte-derived E2 in reactive astrogliosis, microglial activation, and neuroprotection following an ischemic injury to the brain.
Following cerebral ischemia, reactive astrocytes express the enzyme aromatase and produce 17β-estradiol (E2), although the precise role of astrocyte-derived E2 is poorly understood. In this study, we generated a glial fibrillary acidic protein (GFAP) promoter-driven aromatase knock-out (GFAP-ARO-KO) mouse to deplete astrocyte-derived E2 and elucidate its roles after global cerebral ischemia (GCI). The GFAP-ARO-KO mice exhibited significantly attenuated reactive astrogliosis, as well as enhanced microglial activation, neuronal damage, and cognitive dysfunction after GCI. Transcriptome analysis further revealed that astrocyte-derived E2 was critical for the induction of the JAK-STAT3 signaling pathway, as well as the A2 reactive astrocyte phenotype after ischemia. Collectively, these findings indicate that astrocyte-derived E2 has a key role in the regulation of reactive astrogliosis, microglial activation, and neuroprotection after cerebral ischemia.
Aging‐related tau astrogliopathy (ARTAG) is defined by the presence of two types of tau‐bearing astrocytes: thorn‐shaped astrocytes (TSAs) and granular/fuzzy astrocytes in the brain of old‐aged ...individuals. The present study is focused on TSAs in rare forms of ARTAG with no neuronal tau pathology or restricted to entorhinal and transentorhinal cortices, to avoid bias from associated tauopathies. TSAs show 4Rtau phosphorylation at several specific sites and abnormal tau conformation, but they lack ubiquitin and they are not immunostained with tau‐C3 antibodies which recognize truncated tau at Asp421. Astrocytes in ARTAG have atrophic processes, reduced glial fibrillary acidic protein (GFAP) and increased superoxide dismutase 2 (SOD2) immunoreactivity. Gel electrophoresis and western blotting of sarkosyl‐insoluble fractions reveal a pattern of phospho‐tau in ARTAG characterized by two bands of 68 and 64 kDa, and several middle bands between 35 and 50 kDa which differ from what is seen in AD. Phosphoproteomics of dissected vulnerable regions identifies an increase of phosphorylation marks in a large number of proteins in ARTAG compared with controls. GFAP, aquaporin 4, several serine‐threonine kinases, microtubule associated proteins and other neuronal proteins are among the differentially phosphorylated proteins in ARTAG thus suggesting a hyper‐phosphorylation background that affects several molecules, including many kinases and proteins from several cell compartments and various cell types. Finally, present results show for the first time that tau seeding is produced in neurons of the hippocampal complex, astrocytes, oligodendroglia and along fibers of the corpus callosum, fimbria and fornix following inoculation into the hippocampus of wild type mice of sarkosyl‐insoluble fractions enriched in hyper‐phosphorylated tau from selected ARTAG cases. These findings show astrocytes as crucial players of tau seeding in tauopathies.
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